Pulmonary Embolism (PE) Overview

A pulmonary embolism (PE) is a blood clot in the lung. The clot usually forms in smaller vessels in the leg, pelvis, arms, or heart, but occasionally the clot can be large. When a clot forms in the large veins of the legs or arms, it is referred to as a deep venous thrombosis (DVT). The pulmonary embolism occurs when part or all of the DVT breaks away and travels through the blood in the veins and lodges in the lungs.

The clot travels through the vessels of the lung continuing to reach smaller vessels until it becomes wedged in a vessel that is too small to allow it to continue further. The clot blocks all or some of the blood from traveling to that section of the lung. These blockages result in areas in the lung were the disruption of blood flow does no allow the carbon dioxide waste to be delivered to the air sacs for removal (ventilation). Similarly, since blood is blocked to certain portions of the lung, oxygen can not be extracted from these same air sacs (perfusion). The process of matching the lungs ventilation with the blood flow through the lungs is disrupted, resulting in ventilation-perfusion inequalities. In other words, areas in the lung are ventilated (get air) but get no blood to exchange the waste product carbon dioxide with oxygen.

If the pulmonary embolism is large, it may be capable of such a large mismatch, the patient cannot get enough oxygen into the blood and can be become acutely short of breath. In some instances, clots are so large that blood flow is blocked from the right side of the heart entering the lungs. This can result in instantaneous death. In other patients, the mismatch is not so profound, but still causes symptoms, especially when oxygen demand increases (for example, during exercise). Pulmonary infarction (death of lung tissue due to arterial blockage) is unusual because of collateral circulation.